Micromechanical mass flow sensor and method for the production thereof

ABSTRACT

In a mass flow sensor having a layered structure on the upper side of a silicon substrate ( 1 ), and having at least one heating element ( 8 ) patterned out of a conductive layer in the layered structure, thermal insulation between the heating element ( 8 ) and the silicon substrate ( 1 ) is achieved by way of a silicon dioxide block ( 5 ) which is produced beneath the heating element ( 8 ) either in the layered structure on the silicon substrate ( 1 ) or in the upper side of the silicon substrate ( 1 ). As a result, the sensor can be manufactured by surface micromechanics, i.e. without wafer back-side processes.

[0001] The invention relates to a micromechanical mass flow sensor using silicon technology. The invention furthermore relates to a method for manufacturing such a sensor.

[0002] Silicon micromechanics makes possible economical mass production of small sensor chips using semiconductor technology methods. Mass flow sensors having dimensions on the order of a few hundred micrometers are used, for example, in the automotive sector to control air delivery.

[0003] Mass flow sensors using silicon micromechanics that are presently known, for example from DE 195 27 861 A1, encompass a heating element and typically several temperature measurement elements, all of which are patterned out of a metal resistor layer and are positioned on a thin membrane made of dielectric material. The membrane spans a recess in a silicon substrate.

[0004] When a flow of medium, in particular an air flow, flows along the upper side of the membrane, that flow of medium causes cooling of the membrane. That cooling can be evaluated, in particular, using one temperature measurement element placed upstream (relative to the heating element) and a further temperature measurement element placed downstream, the measurement element placed upstream being more greatly cooled than the one placed downstream. Alternatively, it is also possible to determine the cooling of the membrane by measuring the resistance of the heating element. The temperature measurement elements are also resistors, which are made of a material whose resistance is temperature-dependent. One material that is particularly suitable for the heaters and the temperature measurement elements is platinum.

[0005] It is also known from DE 195 27 861 A1, for the manufacture of a membrane sensor of this kind, to begin with a silicon substrate and firstly to deposit on its upper side a membrane layer made of silicon oxide, silicon nitride, or similar materials. A metal layer is then applied onto the entire surface of the membrane layer, and the measurement and heating elements, conductor paths, etc. are patterned out it by photolithography and etching. In a further process step the recess is etched in, proceeding from the back side of the silicon substrate, by means of an etching frame, so that only a frame made of silicon, with the membrane spanned within it, remains. The underside of the membrane layer is then open (except where it covers the frame), i.e. it is bounded by air.

[0006] This known membrane construction produces a thermal insulation, between the resistors patterned out of the metal layer and the substrate, that is ensured by the recess and the air present in it (the thermal conductivity of air or SiO₂ is approximately two orders of magnitude less than that of silicon). Without a recess, a substantial portion of the heat generated by the heating element would not be dissipated by the medium flowing past, but rather would flow away through the membrane layer to the silicon substrate. This heat flowing away laterally and vertically into the substrate would be problematic simply because a predefined working temperature that is also important in terms of measurement sensitivity would be achievable, at best, only by increasing the energy consumption of the heater.

[0007] A lateral heat outflow would be particularly problematic in terms of the measurement elements that are typically positioned laterally next to the heater, since their function is based on the existence of a temperature gradient. This gradient exists between the center of the membrane (heated by the heating element) and the edge of the sensor (which should be as close as possible to ambient temperature). With a lateral heat outflow, to which the direct thermal coupling via the membrane itself does not make a substantial contribution, the temperature measurement elements would also be heated via the substrate. The decreased temperature gradient between heater and measurement elements would result directly in decreased sensor sensitivity. Thermal decoupling in the lateral and vertical directions thus appears indispensable.

[0008] As described, however, the known membrane sensors require for their manufacture a volume-based micromechanical process from the back side of the substrate. This patterning of the membrane is usually accomplished via a KOH etching process in which the wafer is inserted into an etching box. The manufacturing method using bulk silicon micromechanics with an integrated back-side process is very complex and therefore associated with high cost. In addition, the thin membrane, with typical thicknesses of 1 to 2 um, has a tendency toward undesired vibrations because of its low mechanical stability.

[0009] The goal of the present invention is to describe a micromechanical mass flow sensor that can be manufactured with less complexity than before, and that has sufficient sensitivity and mechanical stability. The invention is also intended to describe a corresponding manufacturing method for a sensor of this kind.

[0010] This goal is achieved by way of a sensor according to claim 1, and by way of a method according to the present invention for manufacturing that sensor in accordance with claim 7. Further embodiments of the invention are evident from the other claims.

[0011] The invention produces a micromechanical mass flow sensor

[0012] having a layered structure on the upper side of a silicon substrate,

[0013] the layered structure encompassing an electrically conductive layer out of which at least one heating element is patterned;

[0014] and having a thermal insulation between the heating element and the silicon substrate

[0015] that encompasses a silicon dioxide block

[0016] which is produced beneath the heating element either in the layered structure on the silicon substrate or in the upper side of the silicon substrate.

[0017] The underlying idea of the invention is to achieve the thermal insulation between the (platinum) resistors and the silicon substrate substantially by way of a sufficiently thick silicon dioxide layer that also has adequate mechanical stability. This silicon dioxide layer is embodied as an oxide block, and either is patterned directly into the silicon substrate from the upper side, or is manufactured on the silicon substrate in a corresponding layered structure. In the latter case, a silicon oxide layer is applied on the silicon substrate, and a silicon layer is applied above that, and the silicon dioxide block is produced in the silicon layer.

[0018] The sensor according to the present invention has the advantage that only surface micromechanical processes, i.e. only front-side processes, are necessary for manufacture, so that complex back-side processes can be omitted. Elimination of the KOH etching step from the back side moreover makes possible a miniaturization of the component. A further advantage is a reduction in scratches and particles on the front side of the wafer, which are almost unavoidable in conjunction with handling in the context of back-side processes.

[0019] One particularly advantageous embodiment of this sensor lies in the fact that a plurality of vertical, crevice-shaped cavities are formed in the oxide block. Since the cavities contain air or a vacuum, the thermal insulation is considerably increased by an oxide block patterned in this fashion.

[0020] Configuration of the oxide block in a layered structure, as mentioned previously, opens up the possibility of removing the silicon oxide layer beneath the oxide block and configuring there, toward the silicon substrate, a horizontal cavity that then contributes substantially to the thermal insulation.

[0021] Advantageously, at least one pillar, adjoining the silicon dioxide block at the top and the silicon substrate at the bottom, is positioned in the horizontal cavity and further improves mechanical stability.

[0022] The invention furthermore produces a method for manufacturing a sensor as defined in claim 1 in which

[0023] after patterning of an etching mask, which either is applied on a silicon layer that is applied above a silicon oxide layer on the silicon substrate, or is applied directly on the silicon substrate, approximately vertical grooves are produced by etching in the silicon material located below the etching mask, lands of silicon remaining behind between the grooves;

[0024] the lands are then completely oxidized by thermal oxidation;

[0025] the silicon dioxide block produced in this fashion by surface micromechanics is covered by deposition of a silicon dioxide sealing layer; and

[0026] before deposition and patterning of the electrically conductive layer, a leveling step is performed to produce a smooth surface on the silicon dioxide block.

[0027] The method according to the present invention permits easy manufacture of the sensor, using only method steps that are well known from semiconductor technology. This makes possible the production of a reproducible layer thickness and structure for the oxide block. In particular, in the interest of greater porosity and thus insulation of the oxide block, the silicon layer or the silicon substrate can be undercut beneath the etching mask by isotropic etching. If the etching mask remains on the lands after etching and is later also covered, it is easily possible to generate wider grooves with thinner lands. In general, only a few layers and photolithography steps are necessary according to the present invention.

[0028] One particularly advantageous embodiment of the method according to the present invention lies in depositing a silicon oxide layer for planarization and then planarizing by chemical-mechanical polishing (CMP). This can be performed by means of a silicon oxide auxiliary layer or also with the use of the silicon dioxide sealing layer. The step of leveling the topography, which is necessary because of the thin, sensitive, electrically conductive layer that must later be applied, thereby becomes particularly easy.

[0029] The invention will be explained in more detail below based on exemplified embodiments, with reference to the drawings. The Figures are not to scale.

[0030]FIGS. 1 through 20 depict, in lateral section, several variants of the sensor according to the present invention at various stages of the manufacturing method;

[0031]FIGS. 1, 2 or 3, and 4 through 7 show successive steps in the manufacture of a first exemplary embodiment;

[0032]FIGS. 8 through 10 show successive steps in the manufacture of a second exemplary embodiment that, as shown in FIGS. 11 through 14, is configured as a further variant;

[0033]FIG. 15 shows an embodiment of the manufacturing method that is usable for all the variants;

[0034]FIGS. 16 through 18 and 19 through 20 respectively show successive steps of two embodiments of the manufacturing method usable for leveling.

[0035]FIG. 1 shows a silicon substrate 1 in which grooves 2 have already been produced by etching from the upper side. Similar grooves, albeit only up to approx. 20 μm deep and produced for other purposes, are already known from surface micromechanics. The width of lands 3 separating grooves 2 is selected so that upon subsequent thermal oxidation, lands 3 are in all cases completely oxidized. The state achieved thereafter is depicted for the situation in which the initial width of grooves 2 was selected so that after complete oxidation of lands 3, there remains between them a crevice 4 depicted in FIG. 2. The initial width of grooves 2 can also be selected so that grooves 2 are completely closed up (see FIG. 3).

[0036] In order to produce additional cavities, it is also possible, from the outset, to etch additional grooves transversely to grooves 2. In this case lands 3 exhibit interruptions and are made up, strictly speaking, of columnar land elements.

[0037] Thermal oxidation is a relatively slow surface process that nevertheless results in conformal growth of the oxide even in the grooves. In all cases, the process must be continued until lands 3, typically 1 to 5 μm wide, are each constituted entirely of SiO₂.

[0038] In the state as shown in FIG. 2 or 3, the actual oxide block 5 having the desired low thermal conductivity has already been produced. The discussion hereinafter will always be of the embodiment having crevices 4 as shown in FIG. 2, since oxide block 5 constructed in this fashion yields insulation advantages. The necessary thickness of oxide block 5 is on the one hand to be selected in accordance with requirements, and on the other hand can be decreased by way of vertical and/or horizontal cavities. Typically, a thickness of up to 100 μm will be sufficient. As the thickness of oxide block 5 increases, so does the mechanical stability of the sensor.

[0039] As the next step, an oxide sealing layer 6 is deposited over the entire surface of substrate 1, and over that a nitride layer for moisture rejection, resulting in the state shown in FIG. 4. In the step thereafter, the platinum layer for resistors 8 (heating and measurement elements), pads, etc. is deposited and patterned (cf. FIG. 5). The spacings of resistors 8 are typically only a few um, so that many of them can, if applicable, be positioned next to one another. Instead of platinum, another electrically conductive layer, made in particular of metal, polysilicon, or germanium, can be produced and patterned. Lastly, a protective oxide layer 9 having a thickness of only a few nm can be deposited and patterned (cf. FIG. 6). Deposition and patterning of aluminum conductor paths 10 are performed last, yielding the finished sensor as shown in FIG. 7.

[0040]FIGS. 8 and 9 show a variant of the sensor in which oxide block 5 is produced not in substrate 1 itself, but in a suitable layered structure on its upper side. As shown in FIG. 8, the procedure begins with a substrate 1 on which a (buried) oxide layer 11 and a silicon layer 12 (for example, epitaxially grown polysilicon) are applied. Oxide layer 11 advantageously serves as an etching stop during production of silicon lands 3 and grooves 2. FIG. 9 shows, by analogy with FIG. 2, oxide block 5 generated by complete oxidation of silicon lands 3 by means of thermal oxidation. The further method steps can be completed by analogy with the steps already depicted in FIGS. 4 through 6, so that ultimately the final result shown in FIG. 10 is obtained.

[0041] The variant just depicted can be used with particular advantage as a starting point for a further variant, in which firstly a gas-phase etching step is performed that results in partial removal of oxide layer 11 beneath the patterned silicon layer. A horizontal cavity 13 is thus generated there (cf. FIG. 11), providing additional thermal insulation between platinum resistors 8 and substrate 1. Oxide block 5 is then once again produced in silicon layer 12 by analogy with FIGS. 8 and 9, resulting in the finished state as shown in FIG. 12. The further steps can take place in the manner already described, resulting in the final state shown in FIG. 13. The necessary thickness of oxide block 5 can be reduced by, or to, a fraction of its value by way of the additional cavity 13.

[0042]FIG. 14 shows a completed sensor 1 that conforms to the embodiment shown in FIG. 13, except that at least one pillar 14, adjoining oxide block 5 at the top and substrate 1 at the bottom, is additionally positioned in horizontal cavity 13. In terms of the process sequence, this requires that oxide layer 11 be appropriately patterned before the application of silicon layer 12. Pillar 14 makes possible greater mechanical stability for this sensor configuration, so that oxide block 5 cannot be caused to vibrate by the air flow being measured.

[0043]FIG. 15 shows a particularly favorable procedure for producing wider grooves 2 with thinner lands 3. In this, etching mask 15 (a hard mask, in particular SiO₂) that remains behind after the etching of grooves 2 is not removed, but instead remains on substrate 1 during subsequent thermal oxidation. Trenching processes with stronger undercutting caused by isotropic etching, as evident in FIG. 15, are advantageous in this context. In this case, etching mask 15 persists after the trenching process, with an opening that is smaller than the groove width. This makes it possible, after oxidation of lands 3, to seal up even wide grooves 2 or crevices 4, for example using an SiO₂ sealing layer. The wide grooves 2 or crevices 4 allow a greater porosity, with the corresponding insulation advantage, to be produced.

[0044] As shown in FIG. 16, covering oxide block 5 e.g. with oxide sealing layer 6 does not immediately yield a flat surface. As has been ascertained in SEM investigations, depressions 16 occur above crevices. This undesirable topography of only a few micrometers is nevertheless highly problematic in terms of the metal layer that must be applied later. The typical thickness of the platinum layer is approximately an order of magnitude thinner than the topography, so that a considerable risk of breakage would exist.

[0045]FIG. 17 shows one possibility for leveling the topography depicted in FIG. 16. Firstly a nitride layer 7 is deposited onto oxide sealing layer 6; it is evident that this allows depressions 16 to persist. An SiO₂ auxiliary layer 17 is then applied onto nitride layer 7 for subsequent planarization. Planarization of auxiliary layer 17 is accomplished, by means of the chemical-mechanical polishing (CMP) method known per se, down to nitride layer 7 which here serves initially as a planarization stop. A residual oxide 18 remains behind in depressions 16, and planarizes the surface as is evident from FIG. 18. Platinum structures, protective oxide layers, and aluminum conductor paths can then be deposited and patterned as already described above.

[0046] Another possibility for leveling is shown in FIG. 19. In this variant, nitride layer 7 is deposited before oxide sealing layer 6. This eliminates the need for subsequent deposition of oxide auxiliary layer 17 described above.

[0047]FIG. 19 shows the deposited nitride layer 7 and oxide sealing layer 6 deposited thereonto, which once again has depressions 16. Planarization of the structure down to nitride layer 7, which serves as a planarization stop, can once again be accomplished, as described above, by means of CMP. Residues 18 of oxide sealing layer 6 then remain behind at those points at which the critical depressions 16 were previously located (cf. FIG. 20). To improve adhesion of the platinum structures, nitride layer 7 can be converted back into an oxide layer at the surface, in a manner known per se, using a reoxidation process. Platinum structures, protective oxide layers, and aluminum conductor paths can then be deposited and patterned as described above.

[0048] For leveling, as an alternative to CMP planarization, a passivation layer can be applied onto the surface using the spin-on-glass method. The passivation layer flows and fills depressions 16. A thin oxide layer can then be deposited for adhesion of the platinum structures.

[0049] As further leveling alternatives, BPSG or PSG can be applied using a flow-glass method which (like the spin-on-glass method) is known from the technology of highly integrated circuits. Depressions 16 are filled by the flow process, and a smooth surface forms. A thin oxide layer can then be deposited.

[0050] The topography can also be sufficiently decreased by CVD deposition methods known per se. With suitable deposition parameters, only depressions 16 are filled with SiO₂, and an almost planar surface is established. 

What is claimed is:
 1. A micromechanical mass flow sensor comprising a layered structure on the upper side of a silicon substrate (1), the layered structure encompassing an electrically conductive layer out of which at least one heating element (8) is patterned; and having a thermal insulation between the heating element (8) and the silicon substrate (1) that encompasses a silicon dioxide block (5) which is produced beneath the heating element (8) either in the layered structure on the silicon substrate (1) or in the upper side of the silicon substrate (1).
 2. The sensor as recited in claim 1, wherein a plurality of vertical, crevice-shaped cavities (4) are formed in the silicon dioxide block (5).
 3. The sensor as recited in claim 1 or 2, wherein a silicon oxide layer (11), and above it a silicon layer (12), are applied on the silicon substrate (1); and the silicon dioxide block (5) is produced in the silicon layer (12).
 4. The sensor as recited in claim 3, wherein the silicon oxide layer (11) beneath the silicon dioxide block (5) is removed and a horizontal cavity (13) facing toward the silicon substrate (1) is formed there.
 5. The sensor as recited in claim 4, wherein at least one pillar (14), adjoining the silicon dioxide block (5) at the top and the silicon substrate (1) at the bottom, is positioned in the horizontal cavity (13).
 6. The sensor as recited in one of claims 1 through 5, wherein a silicon dioxide sealing layer (6) and a silicon nitride layer (7) are applied between the silicon dioxide block (5) and the patterned electrically conductive layer.
 7. A method for manufacturing a sensor as recited in claim 1, wherein after patterning of an etching mask, which either is applied on a silicon layer (12) that is applied above a silicon oxide layer (11) on the silicon substrate (1), or is applied directly on the silicon substrate (1), approximately vertical grooves (2) are produced by etching in the silicon material (1, 12) located below the etching mask, lands (3) of silicon remaining behind between the grooves (2); the lands (3) are then completely oxidized by thermal oxidation; the silicon dioxide block (5) produced in this fashion by surface micromechanics is covered by deposition of a silicon dioxide sealing layer (6); and before deposition and patterning of the electrically conductive layer, a leveling step is performed to produce a smooth surface on the silicon dioxide block (5).
 8. The manufacturing method as recited in claim 7, wherein the etching mask is applied on the silicon layer (12); and before oxidation of the lands (3), the silicon oxide layer (11) is at least partly removed by way of a gas-phase etching step in order to produce a horizontal cavity (13) beneath the patterned region of the silicon layer (12).
 9. The manufacturing method as recited in claim 8, wherein, in order to produce at least one pillar (14) in the cavity (13), the silicon oxide layer (11) is patterned before application of the silicon layer (12).
 10. The manufacturing method as recited in claim 7, wherein the silicon layer (12) or the silicon substrate (1) is undercut beneath a hard mask (15) by isotropic etching; and the hard mask (15) remains behind on the lands (3) after etching, and is later also covered.
 11. The manufacturing method as recited in one of claims 7 through 10, wherein there is deposited on the silicon dioxide sealing layer (6) a nitride layer (7), onto which a silicon oxide auxiliary layer (17) is deposited for planarization and is then planarized down to the nitride layer (7).
 12. The manufacturing method as recited in one of claims 7 through 10, wherein firstly a nitride layer (7) and then a silicon dioxide sealing layer (6) are deposited onto the oxidized structures in the silicon layer (12) or in the silicon substrate (1), and are then planarized down to the nitride layer (7).
 13. The manufacturing method as recited in one of claims 11 through 12, wherein the planarization is accomplished by chemical-mechanical polishing (CMP) of the silicon oxide auxiliary layer (17) or of the silicon dioxide sealing layer (6), the nitride layer (7) serving as a planarization stop.
 14. The manufacturing method as recited in one of claims 7 through 10, wherein leveling is accomplished by way of a spinon-glass method or a flow-glass method. 